This invention relates to a flat spring member for use in attaching a flat or tubular member to an object. More particularly, this invention relates to a flat spring hose clamp for use in attaching an automobile tubular part and a method for manufacturing it.
The flat spring hose clamp of this invention is superior to that of the prior art with respect to strength after heat treatment and resistance to brittle fracture which is sometimes experienced during manufacture, transportation, and use of it.
A flat spring hose clamp, in particular that for use in automobiles, is used to fix a tubular member such as a hose, pipe, or wire to an object which is also an automobile part.
A typical flat spring hose clamp for use in automobiles is schematically shown in FIGS. 1 and 2, in which a flat spring hose clamp 10 for use in automobiles consists of a plate spring 2 and mating parts 1, 1' provided at the both ends of the plate. The plate spring 2 is bent so that the both ends cross each other. If necessary, a punched through-hole 3 as shown in FIG. 2 may be provided along the longitudinal length of the plate spring 2.
Such articles are manufactured from a high carbon steel such as S30CM-S70CM, or SK7M-SK4M and a low alloy high carbon steel such as SCM435 or SCM445, as specified in JIS G 3311 or JIS G 4802. These high carbon steels and low alloy, high carbon steels are, if necessary after hot rolling and descaling by pickling, subjected to cold rolling as well as spheroidizing by heating the steel at a temperature of around the Ac.sub.1 point for an extended period of time in order to improve accuracy of the thickness of the sheet and its formability including its suitability for blanking, bending, and pressing. After forming into a final shape, as shown in FIGS. 1 and 2, heat treatment such as quenching and tempering, austempering, and the like is performed to strengthen the article being manufactured, resulting in improvement in hardness, tensile strength, and spring properties.
Therefore, steel sheet for use in manufacturing these articles must exhibit a relatively high degree of strength and hardness as well as spring properties only after heat treatment. For this purpose it is necessary to employ steel with a high carbon content (C: not smaller than 0.50%). Since these properties of the final articles of such a high carbon steel are influenced by heat treatment conditions, and especially the tempering temperature, a tempering temperature of up to 500.degree. C., usually in the range of 200.degree.-450.degree. C. is carefully chosen depending on the levels of properties, including hardness, tensile strength, and spring action, required for the article.
However, the before-mentioned thin steel sheets of the high carbon steel specified in JIS suffer from an increase in size of austenitic grains and precipitation of phosphorous along austenitic grain boundaries in the course of heat treatment. Therefore, sometimes brittle fracture takes place for a hose clamp adjusted to a strength level of 150 kgf/mm.sup.2 or higher, even if the heat treatment conditions are determined carefully.
A typical conventional flat spring hose clamp is shown in FIGS. 1 and 2. It consists of a plate spring 2 in a bent form to provide a clamp portion and a pair of mating parts 1, 1'. These have been manufactured by using high strength steel plate with a T.S. of 150 kgf/mm.sup.2 or higher, obtained by applying austempering to a high carbon steel (S50CM, SK5M, SK7M etc.) containing 0.50-0.85% of carbon, since a strong degree of spring action is required. However, since it has a high degree of strength, cracking due to embrittlement occurs in an area where stress is concentrated, during either shelf storage or use under tension.
In order to avoid brittle fracture it is necessary to suppress an increase in strain, due to a high content of carbon, during heat treatment such as quenching, tempering, austempering and the like. For this purpose, it is advisable to employ a CrMo system, low-alloy steel, such as SCM435, SCM445 etc. that contains an amount of carbon reduced to 0.50% or smaller.
Furthermore, it is also advisable to prepare a fine austenitic crystal grain structure and to prevent propagation of cracking within the steel. For this purpose it is necessary to suitably adjust the chemical composition of the steel including chemical components such as Al and N. In particular, in order to refine the austenitic grain structure it is common to precipitate fine particles of AlN and the like during slab heating or soaking before quenching or austempering.
However, in spite of such efforts to improve the resistance to brittle fracture the level of resistance which has been achieved in the prior art is not enough to satisfy the level of the resistance which is required for up-to-date hose clamps.